The present invention relates to an automatic arc welding method for welding metallic materials having a vertically curved weld line, and more particularly the invention relates to a control method for such welding method.
In the automatic arc welding, the welding must be performed by maintaining the arc length constant irrespective of the flat and arcuated portions of metals to be welded, that is, holding the welding torch at practically a fixed position from the surface of the metals and also at a fixed oblique angle, vertically in many instances, with respect to the surface to be welded and maintaining the linear speed of the welding torch tip at a desired value.
In the past, many different control methods have been proposed for the automatic arc welding of metals having arcuate portions and these methods may be divided into two broad groups, i.e., program control systems and profiling control systems. The program control systems are shown, for example, in U.S. Pat. No. 4,008,384. Such control systems are designed so that the contour of convoluted surfaces to be welded is preliminarily analyzed and the command values representing the angles at which the welding torch is to be positioned are stored on a cam, NC memory device or the like from which the command values are reproduced as the welding is proceeded, and such control systems are disadvantageous in that the required storage and reproduction circuits are generally complicated and high in cost. Also, if the materials to be welded are deformed by the application of pressure during the fitting operation or by the heat during the welding so that the materials change from their initially stored shapes, this will make it difficult to position the welding torch at the proper angle. It is also essential that the positioning of the welding apparatus and the materials relative to each other is accomplished with a high degree of accuracy each time the welding is performed.
On the other hand, profiling control systems are shown, for example, in Japanese Laid-Open Patent Publication No. 52-111847. This system employs two styli which detect the inclination in the direction of the weld line in terms of a difference in height and this difference is converted through an arm into an amount of rotation of a pin which in turn is converted into an electric quantity by a resolver. A torch tilting motor is controlled in a manner that the detected amount of inclination is always reduced to zero and in this way the welding torch is held substantially normal to the sheets to be welded.
However, this type of system requires the use of two styli and a mechanism for detecting the difference in height between the styli with a high degree of accuracy. Also, the sensor as a whole must be held at a predetermined height from the surface to be welded and consequently a vertical motor is controlled by detecting the vertical position with a differential transformer. In any way, a highly precise mechanical system must be used so as to transmit a very small difference between the styli and it is necessary to use the two high precision resolvers as well as the two high precision differential transformer. As a result, the sensor tends to be rigid and bulky and this is detrimental from the standpoint of reducing the weight and size of the apparatus.